The present invention relates to a warm air intake system for an internal combustion engine.
In order to improve warming up performance of an internal combustion engine, and furthermore in order to improve operational performance of an internal combustion engine by improving atomization of fuel contained in the intake fuel-air mixture, particularly when the engine is operated in cold countries or in cold climate, it is known and actually practised to utilize warm air existing around the engine and heated by the heat generated in the engine as the intake air for the engine. A warm air intake system for this purpose generally comprises an intake passage and a change-over valve which selectively connects said intake passage to a cold air source such as the outside atmosphere or to a warm air source such as an air space defined around the engine, wherein the change-over valve is properly changed over so as to supply cold air taken from the cold air source to the intake passage or to supply warm air taken from the warm air source to the intake passage. The change-over valve in such a system is generally so adapted as to be operated by intake manifold vacuum of the engine conducted through a vacuum passage which incorporates therein a control valve which selectively opens a middle portion of the vacuum passage to the atmosphere so that the vacuum supplied to the diaphragm chamber is controlled by the on-off operation of the control valve and therefore the position of the change-over valve is determined by the on-off operation of the control valve. A control valve for this purpose generally employs a thermostat valve which opens or closes or valve port which is connected with a middle portion of the vacuum conducting passage in accordance with the temperature of the air supplied to the engine through the intake passage.
A conventional warm air intake system generally has a structure as shown in FIG. 1. In FIG. 1, 1 designates an intake manifold of an engine E which takes in air through a carburetor 2 and an air cleaner 3 attached to the inlet portion of the carbureter. The air cleaner 3 has an annular filtering element 4 which filters air flowing radially inwardly therethrough from its outside to its inside. At a peripheral portion of the housing of the air cleaner 3 is connected an air intake passage 5 which is selectively connected to a passage 7 or to a passage 8 by way of a change-over valve 6. The passage 7 is directly opened to the atmosphere so as to take in relatively cold atmospheric air, while on the other hand the passage 8 is adapted to open in the vicinity of the engine housing so as to take in warm air heated by the heat generated in the engine. The change-over valve 6 has a pivot shaft 9 and a damper 10 pivotably supported by the pivot shaft 9. The damper 10 is operated by a diaphragm means 12 by way of a link 11 which has one end pivotably connected with the damper 10 and the other end supported by a diaphragm 13 of the diaphragm means 12. The diaphragm 13 is resiliently driven downward in the figure by a compression coil spring 14 so that when vacuum of a predetermined level is supplied to a diaphragm chamber 15, the damper 10 is shifted upward as shown in the figure so as to connect the intake passage 5 to the warm air supply passage 8, while in contrast when vacuum supplied to the diaphragm chamber 15 is reduced or cancelled, the damper 10 is driven downward in the figure by the compression coil spring 14 so that the intake passage 5 is connected to the cold air supply passage 7. The diaphragm chamber 15 is connected to the intake manifold 1 by a vacuum passage 16-17 so that the diaphragm chamber is supplied with manifold vacuum. However, a passage 18 is branched from a middle portion of the passage 16-17 so that the middle portion is connected to a thermostat valve 19 provided in the central chamber space of the air cleaner 3 where the thermostat valve is exposed to the flow of air conducted through the intake passage 5 and the air cleaner 3. The thermostat valve 19 is adapted to selectively open the passage 18, that is, a middle portion of the passage 16-17 to the inner chamber space of the air cleaner, that is, substantially to the atmosphere. Therefore, in accordance with the on-off operation of the control valve 19 the vacuum level in the diaphragm chamber 15 is substantially changed. 20 designates a throttling means provided in the vacuum passage 17 upstream of said middle portion in order to make the on-off operation of the control valve 19 effectively reflect the change of vacuum in the diaphragm chamber 15.
In the conventional warm air intake system of this structure the thermostat control valve 19 is generally a bimetallic valve having the structure as shown in FIG. 2. In this structure a bimetal element 21 supports a valve element 22 by its free end which controls the opening of a valve port 23 which defines an open end of the passage 18. In this arrangement the bimetal element 21 is so adapted that when it becomes hotter due to increase of the temperature of the air the flow of which traverses the bimetal element, it flexes upward as seen in FIG. 2 so as tends to to open the port 23, while on the contrary when the temperature of the intake air lowers, the bimetal element straightens so as to tend to close the port 23. When the port 23 is opened, air is drawn through the port toward the passage 18 by the intake manifold vacuum applied to the passage 18, whereby the vacuum supplied to the diaphragm chamber 15 is attenuated so that the damper 10 is shifted downward in the figure. Then the intake passage 5 is supplied with an increased amount of cold air thereby lowering the temperature of the intake air. On the contrary, if the temperature of the air which traverses the bimetal element 21 is low, the bimetal element straightens so as to drive the valve element 22 downward in the figure to close the valve port 23. Then the diaphragm chamber 15 is effectively supplied with manifold vacuum so that the diaphragm 13 is shifted upward in the Figure thereby causing the damper 10 to turn upward in the Figure so that the intake passage 5 is supplied with warm air thereby increasing intake air temperature. This increase of intake air temperature is then responded to by the thermostat control valve 19 so that the system is then operated in the opposite direction, and thus the engine is operated with properly alternating supply of cold and warm intake air.
However, the conventional warm air intake system incorporating a thermostat control valve such as shown in FIG. 2 has the drawback that the bimetal element 21 and the valve element 22 are susceptible to vibration energized by the flow of air which traverses these movable elements and that, because of this, the operation of the system is unstable and the durability of the system is poor. Furthermore, a thermostat valve of this conventional structure requires a large amount of work for its mounting and adjustment. Furthermore, since a thermostat control valve of the structure as shown in FIG. 2 can provide an intermediate operating condition in which the port 23 is half open, the vacuum in the diaphragm chamber 15 can take a corresponding intermediate value which sets the damper 10 at its intermediate position in which the intake passage 5 is partly connected to the cold air supply passage 7 and is also party connected to the warm air supply passage 8. Such an intermediate condition might be thought to be desirable to provide intake air of a medium temperature. In fact, however, such an intermediate setting of the damper causes instability of the damper and provides unstable control of the warm air intake system.